1. Field of the Invention
This invention relates generally to the aesthetic and environmental beneficiation of coastal sands. More specifically this invention relates to separation of phosphatic materials from coastal sands in existing beaches, or in marine and near shore sand deposit resources, by flotation processes.
2. Description of Related Art
Attractive beaches are valuable assets to coastal communities. Beaches are accumulations of loose sediment along the shoreline. Some beaches have rocks, but the most valuable beaches have sand particles in the range of about 0.1 mm to 1 mm.
The origin of sand is mineral fragments broken from coastal and inland rocks. Wave action and shifting coastal currents abrades the fragments as beaches, barrier islands, and marine and land deposits are formed and reformed over long periods of geologic time.
Most of the beaches along the southeastern and gulf coasts of the United States are desirable light colored to almost white. The rapid erosion that once brought fresh mineral material to the coasts slowed long ago, and only the exceptionally hard, water resistant, clear white quartz has survived eons of wear as rounded sand grains in the 0.1 mm to 0.2 mm diameter range.
Maintaining high quality beaches can be costly. Once polluted, a beach may take decades for natural forces to restore it. Wave and coastal currents, especially during storms, can seriously erode or even eliminate beaches that foster tourism and protected coastal construction. Beaches can be enlarged or restored through a process referred to as beach nourishment or renourishment, which may require importing hundreds of thousands of cubic yards of sand to the affected site. Locating such large deposits of acceptable quality sand is difficult. Because shifting coastal currents make beaches part of a dynamic process, mining some marine sand deposits may cause other beaches to erode. Poor quality sand used to renourish a beach may eventually be carried to other beaches in coastal currents.
Beaches and coastal sands frequently contain shells and shell fragments, and phosphatic materials. Intact shells are often attractive to beach goers, and small amounts of shell fragments are usually not objectionable. But the phosphatic material found in coastal sands on the west coast of Florida, some California beaches, and likely many beaches worldwide, can be objectionable because of its dark color, and high concentrations of phosphorus and fluorine. It is surprising that the benefits of removing these phosphatic materials from beach sands by processes such as flotation have not been recognized.
Phosphorus is the eighth most abundant element in the earth's crust, and deposits of minerals containing the element are widespread. Phosphorus has very limited solubility in seawater, and the element forms deposits in seawater called phosphorites in association with calcium and fluorine. The phosphorite typically contains about 15% of phosphorus (% P) and 2-3% of fluorine (% F). The phosphorite is brown to black in color because organic matter contaminants are present.
Much of the southern Florida peninsula is underlain by a shallow deposit of thick phosphorite that formed in ancient times. This deposit has been disturbed by natural river flow, as well as human activity. Other phosphorite may arrive on the coast from other land and marine deposits. Animals build the inorganic portion of their bones from the mineral hydroxyapatite, Ca (5) PO (4) (3) OH, and fossilized bone is common both onshore and offshore. Fossilized bone contains about 15% of phosphorus, and strongly absorbs fluorine from the environment until it contains about 1% of fluorine. Fossilized bone is also often dark from organic impurities.
Thus phosphorites and fossil bone have about identical negative effects on coastal sands, and both are included herein in the term “phosphatic material”. The presence of small grains of phosphatic material darkens a beach: the presence of larger particles or pebbles of phosphatic material gives the appearance of contamination with charcoal or partially burned items. The phosphatic material contains potentially toxic concentrations of fluorine if ingested. When the phosphorus is solubilized, it can contribute to excessive growth of algae, and to the organisms that cause highly destructive coastal red tides.
Phosphatic materials are dissolved by acid in the stomach, but contain far too much fluorine for use as an animal feed phosphorus supplement. A ½ cm diameter pebble of phosphatic material contains about 7 mg of fluorine. The toxic dose of fluorine varies widely by animal species. The LD50 (the 50 percentile lethal dose for animals) for fluorine is about 32 mg per kg for humans. Adult humans would not be at risk, although a child ingesting an attractive shiny black pebble of phosphatic material could become ill. The LD 50 for dogs is only 4mg per kilogram, but ½ mg per kilogram causes visibly toxic effects. The LD50 for wild birds is estimated at about 50 mg per kilogram, but many birds must consume stones for grit to aid digestion, and such birds might be at risk of cumulative fluorine poisoning from phosphatic material.
The concentration of phosphorus in surface ocean water is very low, on the order of 0.01 mg per liter of water. The low concentration of phosphorus is often the limiting nutrient for growth, and the resulting small number of algae and other small organisms assures water clarity. An increase in the number of upper surface organisms can cause death of bottom life by shading, or by oxygen depletion if the organisms at the surface settle, die and decay. The deterioration of many coastal bays and estuaries has been blamed on increased phosphorus flows from farmlands and lawns, septic tanks and sewage. The increased flow of phosphorus from land sources is also suspected by many to be responsible for the increasing number of red tide incidents as well as their severity and duration.
Red tides occur on all coasts of the United States and many other regions of the world, but in Florida they are very frequent. The organism that causes the red tide reproduces rapidly and produces potent toxins that litter beaches with dead fish. If the wind is blowing toward shore, the toxin in the air causes severe respiratory problems for persons on or near the beach. The factors that cause the rapid growth of the red tide organisms are not well understood. Even if an increased phosphorus concentration is not the direct cause of a red tide incident, an increase in phosphorus is needed to sustain the rapid growth.
A red tide may involve a hundred or more square miles of sea surface, and a large quantity of phosphorus would be required to significantly increase phosphorus concentration in the top foot of sea surface. For example, to increase the phosphorus concentration from 0.01 mg per liter to 0.02 mg per liter in the top foot of 100 square miles of ocean would require about 1,740 pounds of phosphorus. Land sources of phosphorus are suspect because the red tides usually occur in summer when heavy rains cause runoff into coastal waters.
However, coastal sands containing phosphatic material could also release very large amounts of phosphorus during summer rains and storms. A mile of beach with a million cubic yards of sand containing a pound of phosphatic material per ton of sand contains about 225,000 pounds of phosphorus in 1.5 million tons of sand; that is the amount of phosphorus contained in more than 17 million pounds of a typical lawn fertilizer containing 3% of phosphorus reported as P205 (3% P205=1.32% P). Three of four sand samples from Florida west coast beaches examined contained more than one pound of phosphatic material per ton of sand.
The phosphatic material is poorly soluble in seawater, but yields solutions of about 1 mg of phosphorus per liter of rainwater or groundwater, and several ppm of phosphorus in water made acidic by decaying organic matter. In addition, the relatively soft phosphatic material is subject to abrasion by the hard silica sand by wave action and burrowing animal activity. The very fine abraded material may supply not only phosphorus to red tide organisms, but a supply of soluble iron as well. Coastal sands in offshore deposits and sand in coastal currents may also supply substantial phosphorus from phosphatic materials to seawater, especially when disturbed during violent storms.
Costly efforts to reduce phosphorus runoff from the land have been made in many coastal communities. Efforts to further restrict phosphorus fertilizer use may prove poorly cost effective and damage lawns and reduce crop yields. Net flow of phosphorus into offshore waters might be more economically achieved by removing phosphatic materials from coastal sands. Sand for renourishing beaches would be most economically freed of phosphatic material during needed handling and transportation, but existing beaches could also be processed. Possibly it would prove cost effective to remove phosphatic material from some marine sand deposits even if no immediate use of the sand was available. The recovered phosphatic material might have some value, for example in conventional fertilizer production or as a slow release phosphorus fertilizer. This disclosure demonstrates that the phosphatic materials can be separated from the sand by a froth flotation process.